1. Field of the Invention
The present invention relates to a phosphorescent compound and an organic light emitting diode (OLED) device and more particularly to a phosphorescent compound having improved emitting efficiency due to high triplet energy and an OLED device using the same.
2. Discussion of the Related Art
Recently, requirement for flat panel display devices having small occupied area is increased. Among the flat panel display devices, an OLED device, which may be called to as an organic electroluminescent device, is widely introduced.
The OLED device emits light by injecting electrons from a cathode as an electron injection electrode and holes from an anode as a hole injection electrode into an emission compound layer, combining the electrons with the holes, generating an exciton, and transforming the exciton from an excited state to a ground state. A flexible substrate, for example, a plastic substrate, can be used as a base substrate where elements are formed. Since the OLED device does not require a backlight assembly, the OLED device has low weight and low power consumption. Moreover, the OLED device can be operated at a voltage (e.g., 10V or below) lower than a voltage required to operate other display devices. In addition, the OLED device is adequate to produce full-color images.
A general method for fabricating OLED device will be briefly explained below.
(1) First, an anode is formed on a substrate by depositing a transparent conductive compound, for example, indium-tin-oxide (ITO).
(2) Next, a hole injecting layer (HIL) is formed on the anode. For example, the HIL may be formed of 4,4′-bis[N-[4-{N,N-bis(3-methylphenyl)amino}phenyl]-N-phenylamino]biphenyl (DNTPD), which is represented in following Formula 1-1, and have a thickness of about 10 nm to about 60 nm.
(3) Next, a hole transporting layer (HTL) is formed on the HIL. For example, the HTL may be formed of 4,4′-bis[N-(1-naphtyl)-N-phenylamino]-biphenyl (NPB), which is represented in following Formula 1-2, and have a thickness of about 30 nm to about 60 nm. In the OLED device using the phosphorescent compound, an electron blocking layer may be formed on the HTL by a thickness of about 5 nm to 10 nm to increase emission efficiency and lifetime.
(4) Next, an emitting compound layer (EML) is formed on the HTL. A dopant may be doped onto the EML. In a phosphorescent type, the EML may be formed of 4,4′-N,N′-dicarbaxole-biphenyl (CBP), which is represented in following Formula 1-3, and have a thickness of about 30 nm to about 60 nm, and tris((3,5-difluoro-4-cyanophenyl)pyridine)irdium(III) (FCNIr), which is represented in following Formula 1-4, as the dopant may be doped to form a blue emitting material pattern. In addition, for displaying full color image, red and green emitting material patterns are formed.
(5) Next, an electron transporting layer (ETL) and an electron injecting layer (EIL) are stacked on the EML. In the OLED device using the phosphorescent compound, a hole blocking layer may be formed before forming the ETL by a thickness of about 5 nm to 10 nm to keep the triplet exciton in the EML.
(6) A cathode is formed on the EIL, and a passivation layer is formed on the cathode.

Recently, a phosphorescent compound is more widely used for the emission layer than a fluorescent compound. The fluorescent compound only uses singlet energy corresponding to about 25% of excitons for emitting light, and triplet energy corresponding to about 75% of excitons is lost as a heat. However, the phosphorescent compound uses not only the singlet energy but also the triplet energy for emitting light. The phosphorescent dopant includes a heavy atom, such as iridium (Ir), at a center of an organic compound and has a high electron transition probability from the triplet state to the single state.
However, the efficiency of the dopant is rapidly decreased because of a quenching phenomenon such that there is a limitation in the emitting material layer of the dopant without a host. Accordingly, it is desired to form the emitting material layer by the dopant with the host having higher thermal stability and triplet energy.
In the OLED device including the phosphorescent compound, a hole from anode and an electron from the cathode combine at the host of the emitting material layer. Energy transition of a singlet exciton from the host into a singlet or triplet energy level of the dopant is generated, and energy transition of a triplet exciton from the host into the triplet energy level of the dopant is generated. The exciton into the singlet energy level of the dopant is transited again into the triplet energy level of the dopant. Namely, all excitons are transited into the triplet energy level of the dopant. The excitons in the triplet energy level of the dopant are transited into a ground state such that the emitting material layer emits light.
For an efficient energy transition into the dopant, a triplet energy of the host should be larger than that of the dopant. When the triplet energy of the host is smaller than that of the dopant, an energy counter-transition from the dopant to the host is generated such that an emission efficiency is reduced. The triplet energy is an important fact in the HTL and the ETL as well as the host and the dopant.
Referring to FIG. 1, CBP, which is widely used for the host, has a triplet energy level of about 2.6 eV, which is larger than that of Firpic or FCNIr, which are used as a blue dopant, such that emission efficiency is reduced.
Accordingly, new blue phosphorescent compound having the triplet energy larger than that of the phosphorescent dopant is required.